[Libguestfs] [PATCH nbdkit] vddk: Suppress errors in can_extents test (RHBZ#1709211).

[Richard W.M. Jones]

In can_extents we test whether the VDDK function
VixDiskLib_QueryAllocatedBlocks works before declaring that extents
are supported.  This VDDK function can fail in several ways but most
notable if the server doesn't support querying extents.

In this case VDDK raises an error message through the error_function
callback, which we turn into a call to nbdkit_error.  In most cases
this will cause something to be printed to stderr and/or logged
noisily to syslog, which is not desirable.

This commit adds a per-thread error suppression mechanism so that we
can temporarily hide errors, especially while doing this test, but I
guess it might be useful in future in other cases too.

Note this only affects a narrow range of versions of VMware ESXi and
VDDK.  In particular to see this problem you would need
VMware ESXi (server) <= 5.5 and VDDK (client) >= 6.7.
---
 plugins/vddk/vddk.c | 80 ++++++++++++++++++++++++++++++++++++---------
 1 file changed, 64 insertions(+), 16 deletions(-)

diff --git a/plugins/vddk/vddk.c b/plugins/vddk/vddk.c
index 641c4d6d..94b3cbc9 100644
--- a/plugins/vddk/vddk.c
+++ b/plugins/vddk/vddk.c
@@ -43,8 +43,9 @@
 #include <fcntl.h>
 #include <libgen.h>
 
+#include <pthread.h>
+
 #define NBDKIT_API_VERSION 2
-
 #include <nbdkit-plugin.h>
 
 #include "cleanup.h"
@@ -76,6 +77,7 @@ int vddk_debug_datapath = 1;
 static void *dl;                           /* dlopen handle */
 static bool init_called;                   /* was InitEx called */
 static char *reexeced;                     /* orig LD_LIBRARY_PATH on reexec */
+static pthread_key_t error_suppression;    /* threadlocal error suppression */
 
 static char *config;                       /* config */
 static const char *cookie;                 /* cookie */
@@ -108,6 +110,35 @@ static bool is_remote;
   if (vddk_debug_datapath)                                      \
     nbdkit_debug ("VDDK call: %s (" fs ")", fn, ##__VA_ARGS__)
 
+/* Load the plugin. */
+static void
+vddk_load (void)
+{
+  int err;
+
+  err = pthread_key_create (&error_suppression, NULL);
+  if (err != 0) {
+    nbdkit_error ("vddk: pthread_key_create: %s\n", strerror (err));
+    exit (EXIT_FAILURE);
+  }
+}
+
+/* Unload the plugin. */
+static void
+vddk_unload (void)
+{
+  if (init_called) {
+    DEBUG_CALL ("VixDiskLib_Exit", "");
+    VixDiskLib_Exit ();
+  }
+  if (dl)
+    dlclose (dl);
+  free (config);
+  free (libdir);
+  free (password);
+  pthread_key_delete (error_suppression);
+}
+
 static void
 trim (char *str)
 {
@@ -133,12 +164,37 @@ debug_function (const char *fs, va_list args)
   nbdkit_debug ("%s", str);
 }
 
+/* If the thread-local error_suppression pointer is non-NULL (we don't
+ * care about the actual value) then we will suppress error messages
+ * from VDDK in this thread.
+ */
+static void
+set_error_suppression (void)
+{
+  static const int one = 1; /* Something that gives us a non-NULL pointer. */
+  pthread_setspecific (error_suppression, &one);
+}
+
+static void
+clear_error_suppression (void)
+{
+  pthread_setspecific (error_suppression, NULL);
+}
+
+static bool
+are_errors_suppressed (void)
+{
+  return pthread_getspecific (error_suppression) != NULL;
+}
+
 /* Turn error messages from the library into nbdkit_error. */
 static void
 error_function (const char *fs, va_list args)
 {
   CLEANUP_FREE char *str = NULL;
 
+  if (are_errors_suppressed ()) return;
+
   if (vasprintf (&str, fs, args) == -1) {
     nbdkit_error ("lost error message: %s", fs);
     return;
@@ -149,21 +205,6 @@ error_function (const char *fs, va_list args)
   nbdkit_error ("%s", str);
 }
 
-/* Unload the plugin. */
-static void
-vddk_unload (void)
-{
-  if (init_called) {
-    DEBUG_CALL ("VixDiskLib_Exit", "");
-    VixDiskLib_Exit ();
-  }
-  if (dl)
-    dlclose (dl);
-  free (config);
-  free (libdir);
-  free (password);
-}
-
 /* Configuration. */
 static int
 vddk_config (const char *key, const char *value)
@@ -878,6 +919,11 @@ vddk_can_extents (void *handle)
     return 0;
   }
 
+  /* Suppress errors around this call.  See:
+   * https://bugzilla.redhat.com/show_bug.cgi?id=1709211#c7
+   */
+  set_error_suppression ();
+
   /* However even when the call is available it rarely works well so
    * the best thing we can do here is to try the call and if it's
    * non-functional return false.
@@ -889,6 +935,7 @@ vddk_can_extents (void *handle)
                                          0, VIXDISKLIB_MIN_CHUNK_SIZE,
                                          VIXDISKLIB_MIN_CHUNK_SIZE,
                                          &block_list);
+  clear_error_suppression ();
   if (err == VIX_OK) {
     DEBUG_CALL ("VixDiskLib_FreeBlockList", "block_list");
     VixDiskLib_FreeBlockList (block_list);
@@ -1027,6 +1074,7 @@ static struct nbdkit_plugin plugin = {
   .name              = "vddk",
   .longname          = "VMware VDDK plugin",
   .version           = PACKAGE_VERSION,
+  .load              = vddk_load,
   .unload            = vddk_unload,
   .config            = vddk_config,
   .config_complete   = vddk_config_complete,
-- 
2.25.0